1. FIELD OF THE INVENTION
The present invention relates to halogen-free filled elastomer blend compositions which are flame retardant and contain an inorganic filler as the major component.
2. DESCRIPTION OF THE PRIOR ART
An "elastomer" is a rubber-like polymer which can be stretched to at least twice its original length and which retracts very rapidly to approximately its original length when the force exerting the stretching is released. An elastomer has an elastic modulus of about 10,000 psi or less and an elongation usually greater than 200% in the uncrosslinked state at room temperature using the method of ASTM D638-72.
A "thermoplastic" material is a linear or branched polymer which can be repeatedly softened and made flowable when heated and returned to a hard state when cooled to room temperature. It generally has an elastic modulus greater than 10,000 psi using the method of ASTM D638-72. In addition, thermoplastics can be molded or extruded into articles of any predetermined shape when heated to the softened state.
A "thermoset" or vulcanizate is a crosslinked polymer which when subjected to heat does not flow or soften.
The term "filler" is used for additives such as organic or inorganic materials which reduce the cost of a composition and may also provide additional characteristics such as imparting fire or flame retardance, stiffening the composition or other such improvements.
Thermoplastic elastomers (TPEs) are a family of materials that have the properties of elastomers but can be processed like thermoplastics. TPEs are generally made by special block copolymerization or graft polymerization or blending of two polymers. In each case the thermoplastic elastomer contains at least two segments, one of which is thermoplastic and the other elastomeric.
An example of a block copolymerized TPE is disclosed in U.S. Pat. No. 3,792,127 which discloses the block copolymerization of styrene and butadiene. The TPE consists of polystyrene blocks and polybutadiene blocks. By itself, polystyrene is a thermoplastic while polybutadiene is an elastomer. Similarly, TPEs can be made by graft polymerization as in U.S. Pat. No. 3,265,765 to Holden et al and in the article by Hartman et al "Butyl Grafted to Polyethylene Yields Thermoplastic Elastomer", Rubber World, pages 59 to 64, Oct. 1970, which discloses thermoplastic elastomers made by grafting an elastomeric segment, polyisobutyl to a thermoplastic material, polyethylene.
Other examples of a blend of a thermoplastic and an elastomer to produce a TPE are disclosed in U.S. Pat. No. 4,130,535 to Coran and U.S. Pat. No. 3,806,558.
The use of fillers in small amounts of less than about 40% by weight, such as talc, mica, titanium dioxide, calcium carbonate, aluminum hydroxide, to reduce the cost or to add rigidity to a product is well known. For example, U.S. Pat. No. 3,965,055 to Shichman et al discloses a vulcanizate prepared from a rubber-resin blend wherein the resin particles are dispersed in the rubber. Other modifying or compounding ingredients such as carbon black, clays, hydrated silica and the like, may be incorporated into the rubber and resin, if desired. However, the composition results in a vulcanizate which by definition is not a thermoplastic but rather a thermoset.
U.S. Pat. No. 4,327,199 to Coran et al discloses thermoplastic compositions comprising neutralized acid-containing acrylic copolymer rubber obtained by blending thermoplastic crystalline polyester therewith. The properties of this composition can be modified with the addition of other ingredients such as carbon black, silica, titanium dioxide, pigments, clay and the like. Typical additions of filler can vary from about 15-80 parts by weight of filler per 100 parts by weight of rubber. Thus, the filler is a minor portion (13-44%) of the total composition.
Some fillers, such as antimony trioxide and chlorinated or brominated compounds are used to impart additional properties such as fire retardance. For example, U.S. Pat. No. 4,275,180 to Clarke discloses a crosslinkable or crosslinked blend of an elastomer and a thermoplastic polymer, such that both the elastomer and the thermoplastic polymer are substantially free of halogen-containing substances. However, all of Clarke's examples include either chlorinated or brominated compounds. Hence the total composition contains halogen-containing compounds which generate hydrochloric acid gas and other toxic and harmful vapors when subjected to high heat and/or an open flame. These vapors make the composition unsuitable for applications in enclosed spaces such as high rise buildings and military and aerospace vehicles. Moreover, Clarke's blend contains only minor amounts of filler, usually not more than 40% by weight.
U.S. Pat. No. 4,108,962 to Pedlow discloses flame retardant external tape coverings that are used for covering polyolefin insulating material. The resinous base substance of the tape composition can be a halogen-containing thermoplastic.